Thermionic cathode and direct current heater assembly



Aug. 12, 1958 'J. E. BEGGS ETAL 2,

THERMIONIC CATHODE AND DIRECT CURRENT HEATER ASSEMBLY Filed June 2. 1955 44- 1 Cal/rode Temp. Opera f/hg flange I I5 I I Temper-a fare C.

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United States Patent THERMIONIC CATHODE AND DIRECT CURRENT HEATER ASSEMBLY James E. Beggs and Harold F. Webster, Schenectady,

N. Y., *assignors to General Electric Company, a corporation of New York Application June 2, 1955, Serial No. 512,723

9 Claims. (Cl. 313-3157) Our invention relates to improved thermionic cathode and direct current heater assemblies and particularly to such assemblies which utilize thermoelectric junction heating.

In accordance with the prior art, a great variety of resistance heater elements have been provided for indirectly heated cathodes. it has been a problem in connection with most heaters of this type to generate the heat in close proximity to the surface to be heated to electron emissive temperature. We have found that it is possible to generate the heat for elevating the temperature of the cathode very close to the emissive region by forming a thermoelectric junction in close proximity to or as a part of the cathode. It is well known that junctions between metals or alloys having different thermoelectric properties may operate to generate or absorb heat depending upon the direction of current through the junction. In accordance with an important feature of our invention, we provide an improved heater-cathode assembly which is characterized by the small amount of heat required for raising the cathode to emission temperature and by the generation of that heat essentially at the emissive surface.

Further objects and advantages of our invention will become apparent as the following description proceeds, reference being had to the accompanying drawing and its scope will be pointed out in the appended claims. in the drawing, Fig. l is an elevational view in section showing 'a cathode-heater assembly incorporated in an electric discharge device of the type for which our invention is particularly adapted. Fig. 2 is an elevational View in section illustrating a modified form of our invention in which the heater is completely insulated from the cathode support, Fig. 3 is an elevational view in section of a still further modification, and Fig. 4 is a group of curves show ing the thermal electromotive force versus temperature characteristic for a number of materials with respect to platinum.

Referring now to Fig. l of the drawing, we have shown our invention embodied in a triode electric discharge device of the disc seal type in which the envelope is made up of alternate metal rings or terminals and insulating rings or spacers. As shown in the drawing, the anode ll) is supported from a disc-like terminal 11; the grid 12 is connected to and supported from a ring-like metal terminal 13; the cathode terminal is provided by a similar ring 14; and the heater terminal by a disc 15. The anode and grid terminals are separated by insulating spacer 16, the grid and cathode terminals by an insulating spacer 17, and the cathode and heater terminals by the insulating disc 18.

As illustrated, the cathode includes a thin cylindrical metal foil cylinder 1) which is closed at the end thereof The surface of the disc adjacent the grid is coated with a suitable electron emissive material 21 which may, for example, be prepared from a suspension of one or more of the alkaline earth (barium, calcium, strontium) carbonates,

in accordance with practices well known in the art. The foil cylinder is bonded to the insulating ring 17 and connected to the cathode terminal 14 by means of a metallized layer 22 which may be formed on the lower surface of the insulator 17 between the cathode shell and the terminal 14. A circuit between the heater terminal 15 and the central region of the disc or cathode base member Ell is provided by the wire 23. The wire passes through an opening in the insulating member 18 and is joined to the terminal 15.

In accordance with our invention, the materials of the cathode and heater assembly just described are chosen to provide one or more hot thermoelectric junctions at the base member Zll. While a number of combinations of materials may be used as long as they have the proper thermoelectric characteristics with respect to the voltage to be applied to the heater circuit, they must be chosen not only for these characteristics but also for their shit ability for use in vacuum tubes as cathode and heater parts. A combination which has been very effective is one in which the wire 23 is of palladium, the disc 24 of platinum, and the cylindrical foil of titanium. The terminals 1d and 15 may be made of the same material or, if of different materials, they should have similar thermoelectric characteristics. Copper or titanium are particularly suitable materials for these terminals. With this combination of materials in the heater circuit, the terminals 14 and 315 are connected to the terminals of a direct current heater supply voltage with terminal 15 at a negative voltage with respect to the terminal 14. This provides two hot junctions, one at the junction between the palladium wire and the platinum disc 20 and the other at the junction between the periphery of the disc 20 and the titanium foil support 19. Titanium foil is a good material from the standpoint of the small amount of heat conducted from the active surface of the cathode. It is a very poor conductor of heat and when made of a thickness of the order of 0.5 mil, the heat lost from the cathode surface through the support is very small. Another cornbination of materials which works satisfactorily is one in which the wire 23 is a platinum wire, the disc 21 is titanium and the foil i titanium. Hzafnium may be used in place of titanium and has thermoelectric properties essentially the same as those of titanium. it is not essential that two hot junctions be formed and the disc 2t) and support 19 may be of the same material, titanium, for example, and the wire 23 may be platinum or palladium.

The joint between the wire 23 and the disc 20 as well as between the disc and the tantalum shield 19 may be formed by welding. It is also possible to make soldered joints at the junctions without destroying the heat generating properties of these junctions, providing the amount of solder utilized is small and that the alloying is not suificient to destroy the basic characteristic of the junction. As indicated above, heaters of this type are particularly useful in connection with cathodes requiring a small amount of heat. The construction illustrated in Fig. 1 lends itself particularly to this type of heater in combination with the cathode assembly since the total heat requirements may be small and it is relatively easy to form the junctions very close to the emissive surface. A cathode heater in accordance vn'th Fig. 1 has been constructed in which the heater power is in the order of .2 of a watt. It will be understood that one or more cold junctions are present in the circuit of the heater. In Fig. 1, for example, there is a cold junction between terminal 15 and wire 23 and also between 19 or 22 and 14.

In Pig. 2, we have shown a modification of our invention utilizing an insulated heater assembly. The discharge device, as illustrated, is generally the same as that shown in Fig. 1 and corresponding parts have been designated with the same reference numerals. Referring now to Fig. 2, the heater terminal 15 has been replaced by two generally semicylindrical terminals 15 and 1 which are bonded in spaced and mutually insulated relation on the outer surface of the disc 18'. The cathode assembly includes a cylindrical support 19 which is closed at its upper end by a thin ceramic disc 24. The disc may to advantage be a high purity alumina ceramic. The upper surface of the disc 24 may be formed integrally with the foil support 19 or it may comprise a thin metal coating which is bonded to the ceramic in accordance with methods well known in the art. In either case, the emissive coating 25 is applied to the metal surface. The heater assembly includes conductors 26 and 27 connected respectively at one end with the terminals and 15" and terminating at the upper end in junctions 29 and 33 formed between them and a metal strap 31. in accordance with the considerations previously discussed, the materials of the conductors 26, 2'7 and the connecting strap 31 are chosen to provide two hot junctions, provided the polarity of the voltage applied to the heater terminals 15 and 15" is suitable. For example, conductor 26 may be of molybdenum, the conducting member 31 of titanium, and conductor 27 of palladium. In such a case, the terminal 15 will be made the positive terminal of the direct current heater supply voltage.

It will be apparent that other combinations of materials may be connected in series between the heater terminals to provide the desired hot junctions adjacent the emissive surface. It will be understood that the materials are arranged in series in a descending order of the magnitude of their positive thermoelectric characteristics with the most positive connected to the positive terminal. We have shown in Fig. 4 the thermoelectric characteristics of a number of materials with respect to platinum with the operating temperature range for oxide cathodes also indicated. In this figure, curves A-I, inclusive, represent the following materials: (A) 90% nickel-10% vanadium, (B) molybdenum, (C) tungsten, (D) titanium or hafnium, (E) tantalum, (F) platinum, (G) palladium, (H) nickel, and (I) 90% nickel-10% copper alloy.

In Fig. 3, we have shown an elevational view in section of a cathode heater assembly similar to that shown in Fig. 2 but with only a single junction. Parts corresponding to those of Fig. 2 have been designated by the same reference numerals. As illustrated in Fig. 3, a single junction between conductors 2'7 and 26' is formed by overlapping portions of these conductors. In contact with the lower surface of the ceramic disc 24, the conductors may be selected in accordance with the same considerations that have been previously discussed and may, for example, be made of molybdenum and nickel. It will be understood that the molybdenum wire will be connected to the positive heater terminal. It will be apparent that the cathode and heater may be formed by applying a coating directly to the upper surface of the junction formed by the overlapped surfaces of the wire 26' and 27', in which case the wire 26 will be made of a metal suitable for a cathode base, such as nickel.

While we have shown and described a particular embodiment of our invention, it will be apparent to those skilled in the art that modifications may be made without departing from our invention and we, therefore, aim by the appended claims to cover any such modifications as fall within the true spirit and scope of our invention.

What we claim as new and desire to claim by Letters Patent of the United States is:

1. A thermionic cathode and direct current heater assembly comprising a conducting disc forming a cathode base member, an electron emissive coating on one side of said disc, a metal foil cylinder supporting said disc and bonded to the periphery thereof at one end of said cylinder, an elongated conductor bonded at one end to a central portion of said disc on the side thereof opposite said coating and providing a thermoelectric junction on said disc, a terminal connected to the other end of said conductor and a second terminal connected with said foil support so that said conductor, said disc and said foil are in series between said terminals, the materials of said conductor, disc and foil having successively more positive thermoelectric characteristics in one direction around the circuit between said terminals, the terminal connected to the more positive thermoelectric material providing the connection for the positive terminal of a direct current heater supply voltage.

2. A thermionic cathode and direct current heater assembly comprising a conducting member forming a cathode base, an electron emissive coating on one side of said member, a thin hollow metal support bonded to the periphery of said conducting member, an elongated conductor bonded at one end to a central portion of said member on the side thereof opposite said coating and providing a thermoelectric junction on said member, a terminal connected to the other end of said conductor and a second terminal connected with said metal support so that said conductor, said member and said support are in series between said terminals, the materials of said conductor, member and support having successively more positive thermoelectric characteristics in one direction around the circuit between said terminals, the terminal connected to the more positive thermoelectric material providing the connection for the positive terminal of a direct current heater supply voltage.

3. A thermionic cathode and direct current heater assembly comprising a conducting disc of titanium forming a cathode base member, an electron emissive coating on one side of said disc, a tantalum foil cylinder supporting said disc and bonded to the periphery thereof at one end of said cylinder, an elongated conductor of a metal selected from the group consisting of palladium and platinum bonded at one end to a central portion of said disc on the side thereof opposite said coating and providing a thermoelectric junction on said disc, a terminal connected to the other end of said conductor and a second terminal connected with said foil support so that said conductor, said disc and said foil are in series between said terminals, the materials of said conductor, disc and foil having successively more positive thermoelectric characteristics around the circuit between said terminals starting with said conductor, the terminal connected to said conductor providing the connection for the negative terminal of a direct current heater supply voltage.

4. A thermionic cathode and direct current heater assembly comprising a disc of conducting material forming a cathode base member, an electron emissive coating on one side of said disc, 21 metal foil cylinder supporting said disc at one end thereof, a conductor of a material having a more negative thermoelectric characteristic than the material of said disc bonded to a central portion of said disc on the side thereof opposite said coating and providing a thermoelectric junction on said disc and a negative terminal connected to another portion of said conductor.

5. A thermionic cathode and direct current heater assembly comprising a titanium disc forming a cathode base member, an electron emissive coating on one side of said disc, a metal foil cylinder supporting said disc at one end thereof, a conductor of palladium bonded to a central portion of said disc on the side thereof opposite said coating and providing a thermoelectric junction on said disc and a negative heater terminal connected to another portion of said palladium conductor.

6. A thermionic cathode and direct current heater assembly comprising a cathode base member, an electron emissive coating covering one region of said base and a heater circuit for saidbase including at least one thermoelectric junction essentially in contact and in good heat 5 transfer relation with said base, said circuit including a plurality of conductors of difiterent thermoelectric characteristics With successive conductors in one direction around said circuit having successively less positive thermoelectric characteristics to provide only hot junctions in good heat transfer relation with said base.

7. A thermonic cathode and direct current heater assembly comprising a cathode base member, an electron emissive coating covering one region of said base memher, a heater circuit for said base member including a means providing a thermoelectric junction in good heat transfer relation with another portion of said base, said circuit including a plurality of conductors of diflerent thermoelectric characteristics with successive conductors in one direction around said circuit having successively less positive thermoelectric characteristics, a positive heater terminal connected With the conductor of said circuit having the most positive thermoelectric characteristic and a negative heater terminal connected with the conductor of said circuit having the least positive thermoelectric characteristics so that all junctions in said heater circuit in good heat transfer relation with said base are hot junctions.

8. A thermionic cathode and direct current heater assembly comprising a cathode base member, an electron emissive coating coving one region of said base member, a heater for said base member including a thermoelectric junction of molybdenum and nickel in good heat transfer relation With another portion of said base, a positive heater terminal connected with said molybdenum conductor and a negative heater terminal connected with said nickel conductor.

9. A thermionic cathode and direct current heater assembly comprising a cathode base member, an electron emissive coating covering one region of said base member and a heater circuit for said base member including at least one thermoelectric junction essentially in contact and in good heat transfer relation with said base, said circuit including a plurality of conductors of different thermoelectric characteristics with successive conductors in one direction around said circuit having successively less positive thermoelectric characteristics, a pair of heater terminals connected respectively with the free ends of said conductors with the positive terminal connected to the conductor having the most positive thermoelectric characteristic,

References Cited in the file of this patent UNITED STATES PATENTS 2,677,778 Baker May 4, 1954 2,682,511 Cronin June 29, 1954 2,721,372 Levi Oct. 25, 1955 2,727,177 Dailey Dec. 13, 1955 2,727,178 Dailey Dec. 13, 1955 

